Cities can become so much hotter than surrounding areas that they generate their own weather, a Nasa scientist said on Tuesday.

This includes brewing violent thunderstorms that then crash lightning and rain on the city and the land downwind.

Dale Quattrochi, from Nasa's Marshall Space Flight Center, said the urban heat island effect - when cities become hotter than the countryside - had serious consequences, both in weather disturbance and in creating pollution.

His team flew Nasa aircraft over a number of cities, seeking out the heat and plotting maps.

The heat of the night

One city was Georgia, Atlanta, where the difference between city and country rose to 7deg Celsius (12deg F) at times.

They found that in the day, when the air temperature over central Atlanta was 27degC (80degF), the ground temperature was 49degC (120degF). At night, the air was 13degC (55degF) and the ground 24degC (75degF).

The convective storms generated by the dome of hot air over the city arose in five days out of nine during an August study period. The storms broke at the "wrong" time, between 0200 and 0300. Normal thunderstorms generally break in the afternoon.

The intense heat in the city also causes pollution problems, with Atlanta recording 62 days in a row of high ozone last summer.

Lost trees

Between 1973 and 1992, 154,000 hectares (380,000) acres of trees were lost from Atlanta urban areas, a rate of 22 hectares (55 acres) per day.

Dr Quattrochi is working with local charities and authorities to try to combat the worst effects of the heat, by planting trees and installing highly reflective roofs and paving stones. It is thought that well-placed trees can reduce heating and cooling costs for a home by up to 20%.

"By 2025, 80% of the world's people will be living in cities, so we must find what we can do to protect their environment," he told the annual meeting of the American Association for the Advancement of Science (AAAS).

One particularly striking problem thrown up by the research is that the temperature on the roofs of some Atlanta buildings, where most air conditioning units are sited, can reach 77degC (170degF).

"It is very inefficient to put cooling units on roof tops," said Dr Quattrochi.

The scale of the problem is shown by the fact that one sixth of the total energy used in the US is spent on cooling, at a cost of $40bn a year.

Growing longer for less

Dr Marc Imhoff, from Nasa's Goddard Space Flight Center, also studied the effects of heat islands over cities but using satellite images. He combined pictures showing city lights at night, i.e. urban areas, with others showing how green the land was.

He found that the growing season in cities was longer than in surrounding areas: "Vegetation greens up earlier in the spring and takes longer to fade in the fall."

However, the extension to the growing season in urbanised areas did not compensate for the drop in productivity as neighbouring green areas were lost to construction.